17-4PH　stainless　steel　was　aged　at　400　degrees　C　for　a　long　time　after　solution　treatment　at　1040　degrees　C　for　1　h.　Micro　hardness　tester,　high-resolution　transmission　electron　microscopy　and　atom　probe　tomography　(APT)　were　used　to　measure　the　hardness　changes　and　observe　the　evolution　of　G-phase.　It　was　found　that　the　hardness　reached　a　peak　of　472.9　HV　after　aging　for　48　h,　and　then　dropped　to　450.0　HV.　APT　results　showed　that　the　number　density　of　Cu-rich　phase　reached　the　maximum　in　the　sample　aged　for　48　h,　which　was　the　cause　of　the　hardness　peak.　In　the　aging　process,　Ni　and　Mn　first　segregated　around　the　Cu-rich　phase　to　form　Ni-Mn　clusters,　and　then　Si　followed　to　segregate　at　the　Ni-Mn　cluster　location　to　form　Ni-Mn-Si　enriched　region.　When　the　atomic　content　ratio　of　Ni:　Mn:　Si　in　the　Ni-Mn-Si　enriched　region　became　16:6:7,　G-phase　was　formed.　The　nucleation　and　growth　of　G-phase　were　accelerated　by　the　precipitation　of　Cu-rich　phase.